Blender selection in viewport suddenly slow
Slow selection is usually fixed by changing the Selection mode in the System panel of the user preferences.

From the manual page about troubleshooting slow selection:

OpenGL Occlusion Queries (User Preferences)
See User Preferences ‣ System ‣ Selection.
This option defaults Automatic, try setting this to OpenGL Occlusion Queries, since there is a significant performance difference under some configurations.